Liquid cooling device in internal combustion engines and process for manufacturing same

ABSTRACT

The invention relates to an apparatus for liquid cooling of an internal combustion engine ( 11 ) and a process for producing it. The apparatus of the invention comprises a cooling circuit ( 13 ) which comprises at least one cooling channel ( 23, 24, 41 ) for a liquid coolant which is in thermal contact with at least one component ( 12   a,    12   b,    31 ) of the internal combustion engine ( 11 ), wherein a wall of the cooling channel ( 23, 24, 41 ) which comes into contact with the coolant has a microstructured surface having a certain porosity and roughness at least in a subregion. According to the invention, such an apparatus is produced by constructing a cooling circuit for a liquid coolant which has cooling channels which can at least partly be brought into thermal contact with the internal combustion engine, wherein a microstructured surface is produced on at least part of the walls of the cooling channels which come into contact with the liquid coolant.

The invention relates to an apparatus for liquid cooling of internal combustion engines and a process for producing it.

In the operation of internal combustion engines, for example, piston engines such as the diesel engines or four-stroke petrol engines used in vehicles, temperatures of above 2000° C. occur in the combustion chambers of the cylinders. The heat transferred from the cylinders via the cylinder walls to the engine block and the cylinder head poses a threat to other components of the internal combustion engine which have only limited heat resistance and therefore has to be removed as efficiently as possible in order to avoid damage to these components as a result of overheating.

Cooling internal combustion engines by means of air or a liquid as cooling medium is known from the prior art. Liquid cooling using water as cooling liquid is particularly preferred here because the high heat capacity and the low viscosity of water makes efficient cooling of the internal combustion engine possible. In a water-cooled internal combustion engine, cooling channels which form part of a cooling circuit through which the cooling water is conveyed are, for example, located in the walls of the cylinder and/or the cylinder head or in the crankcase. In a section of the cooling circuit located outside the internal combustion engine, there is an air-cooled heat exchanger which in the case of motor vehicles is referred to as the radiator and via which the cooling water gives off the heat taken up in the internal combustion engine to the environment. The cooling liquid usually enters the internal combustion engine in a relatively low-lying region and is conveyed through cooling channels or a cooling jacket of the engine block/crankshaft housing into the cylinder head from where it leaves the internal combustion engine again in a region lying higher up. However, dividing the cooling liquid into two separate circuits by means of a preferably actuatable valve before it enters the engine housing and conveying the stream separately into the cooling channels or cooling jacket of the crankshaft housing and the cylinder head is also known.

Further heat exchangers, compressors and condensers which interact with the heating system or the air-conditioning unit of the motor vehicle can be provided in the cooling circuit. Thus, for example, the heat given off by the internal combustion engine can be used at least partly for heating the passenger compartment. Cooling the hot exhaust gases from an internal combustion engine by means of suitable heat exchangers is also known. Here, the heat energy taken from the hot exhaust gases in the start-up phase can, for example, be used for heating the cooling liquid so that the internal combustion engine through which the liquid flows reaches its optimal operating temperature more quickly. However, cooling of the exhaust gases is particularly advantageous in the case of exhaust gas recycle systems which are now being used in the motor vehicle sector, in order to reduce, firstly, the consumption in the part-load region and, secondly, the emissions from internal combustion engines, especially the emission of NO_(x). Here, a substream, which can usually be controlled by means of valves, of the exhaust gas is recirculated to the intake section of the internal combustion engine. The effects of the recirculation of exhaust gas on the consumption and the reduction in the emission of NO_(x) can be improved further when the recirculated substream of exhaust gas is cooled by means of an exhaust gas cooler.

Circulation of the cooling liquid usually occurs by means of a pump located in the cooling circuit which is usually driven directly by the internal combustion engine via a V-belt, so that a cooling liquid flow dependent on the engine revolutions is generated. For the internal combustion engine to reach its optimal operating temperature quickly, bypassing the cooler by means of a thermostat-controlled valve in the heating-up phase of the internal combustion engine is also known.

Apart from uptake of heat produced in the internal combustion engine by heating of the cooling liquid, partial vaporization of the cooling liquid additionally occurs in particularly hot regions, so that highly effective cooling of the corresponding engine surfaces can be effected by means of the corresponding enthalpy of vaporization.

Nowadays, modern internal combustion engines no longer use only water as cooling liquid, but instead use a liquid which is generally referred to as coolant and comprises water together with further additives, first and foremost additives which serve to protect against freezing and corrosion.

Coolant compositions for the cooling circuits of internal combustion engines as are used in motor vehicles usually comprise water together with alkylene glycols, mainly ethylene glycol and/or propylene glycol and/or glycerol as antifreeze components.

For example the use of not only alkylene glycols but also higher glycols and glycol ethers as antifreeze components is known from EP-A-816467.

Since components of internal combustion engines are subjected to high thermal stresses both as a result of the absolute temperatures prevailing during operation and also as a result of temperature fluctuations, any type and any degree of corrosion represents a potential risk factor which can lead to a shortening of the operating life of the internal combustion engine and too a reduction in its reliability. The many materials used in a modern internal combustion engine, for example cast iron, copper, brass, soft solder, steel and lightweight metal alloys, in particular magnesium and aluminum alloys, result in additional potential corrosion problems, especially at places at which different metals are in contact with one another. A variety of types of corrosion, for example, pit corrosion, steel corrosion, erosion or cavitation can occur particularly easily at these places. Modern coolant compositions, therefore also comprise specific corrosion inhibitors which serve as corrosion protection components. DE-A 195 474 49, EP-A 0 552 988 or U.S. Pat. No. 4,561,990 disclose, for example, antifreezes comprising carboxylic acids, molybdates or triazoles for the cooling water of internal combustion engines. EP-A 0 229 440 describes a corrosion protection component comprising an aliphatic monobasic acid, a dibasic hydrocarbon acid and a hydrocarbon triazole. Specific acids as corrosion protection components are described, for example, in EP-A 0 479 470. Quaternized imidazoles as corrosion protection components are known from DE-A 196 05 509. The coolant compositions also have to be designed so that they are compatible with nonmetallic constituents of the cooling circuit, for example, elastomers and other plastics as hose connections or seals, and do not alter or attack these.

Apart from the steady improvement in the antifreeze and corrosion protection components of a coolant composition, modern technical developments aim at, in particular, an improvement in the cooling properties of the cooling liquid. It has thus been proposed, for example, that the cooling properties be improved by additives which reduce the viscosity and/or the flow resistance of the cooling liquid in the cooling circuit.

The power density which can be achieved by an internal combustion engine is influenced decisively by the efficiency of the liquid cooling. It is therefore an object of the present invention to provide an apparatus for liquid cooling of internal combustion engines, in particular using the above-described liquid coolants, in which the cooling action is improved further, especially surfaces of the internal combustion engine which are subjected to high thermal stresses. The invention also relates to a process for producing such an apparatus.

This object is achieved by the apparatus for liquid cooling of an internal combustion engine having the features of the present claim 1. Advantageous embodiments of the apparatus of the invention are subject matter of the dependent claims.

The invention accordingly provides an apparatus for cooling an internal combustion engine comprising a cooling circuit which comprises at least one cooling channel for a liquid coolant which is in thermal contact with at least one component of the internal combustion engine, wherein a wall of the cooling channel which comes into contact with the coolant has a microstructured surface at least in a subregion. When the present description speaks of a liquid coolant, this refers to the state of matter of the coolant at temperatures of from 0 to 100° C. and atmospheric pressure. Depending on the antifreeze component used, the coolant can also be liquid at lower or higher temperatures.

While attempts are usually made to make the surfaces of the cooling circuit lines which are in contact with the liquid coolant as smooth as possible, in order to minimize the flow resistance of the coolant in cooling circuits known from the prior art for the internal combustion engines, it has now surprisingly been found that a significantly better cooling action can be achieved by means of the cooling apparatus which has been modified according to the invention, without the flow properties of the coolant being adversely affected to an appreciable extent. In particular, it has been found that the microstructured surface of the cooling apparatus of the invention leads both to an improvement in the single-phase heat transfer before the coolant begins to boil and also to a great improvement in the two-phase heat transfer, in particular the readiness of the coolant to boil and the boiling activity in the bubble boiling range. It has thus been found, for example, that superheat of the wall, i.e. the temperature difference between the wall temperature T_(W) and the saturation temperature T_(S) of the coolant at the commencement of bubble boiling, could be reduced from a range from about 20 to 40° C. to a range from about 3 to 10° C.

The apparatus of the invention for liquid cooling of internal combustion engines therefore makes it possible to achieve a decisive improvement in the cooling of the internal combustion engine. Since, as mentioned above, the power density of modern internal combustion engines is frequently limited by the efficiency of heat removal by cooling, the apparatus of the invention also makes it possible to increase the power density of internal combustion engines.

Various components of an internal combustion engine can be cooled by means of the cooling channels having a microstructured surface provided according to the invention. First and foremost the cooling channels are in thermal contact with components of the engine block of the internal combustion engine, for example with the cylinder head and/or the crankcase. However, the term “components of the internal combustion engine” as used in the context of the present invention also comprises components outside the actual engine block, in particular, further heat exchangers such as exhaust gas coolers or oil coolers located in the cooling system of the internal combustion engine. These heat exchangers each have separate cooling liquid circuits; however, they are preferably cooled via subcircuits of the cooling circuit of the internal combustion engine, with the division of the cooling liquid stream into the individual subsections particularly preferably being able to be controlled by means of suitable valves.

In an advantageous embodiment of the apparatus of the invention, the microstructured surface has a mean surface roughness Ra in the range from 1 to 1500 μm, preferably in the range from 20 to 200 μm.

The microstructured surface particularly preferably has a porous structure. The pore size of the porous microstructures is advantageously in the range from 1 to 500 μm. The pore size here relates to the greatest pore diameter in the cross section. The pores can, for example, have an essentially circular cross section, but any other pore geometries are likewise possible. The proportion of pores in the microstructured surface layer can be in the range from 1 to 90%, preferably in the range from 10 to 80% and particularly preferably in the range from 10 to 70%.

The rough and/or porous microstructures of the apparatus of the invention can be distributed regularly or randomly over the surfaces. The preferred pore depth in the case of a random arrangement of the pores corresponds approximately to the pore diameter. Particularly in the case of mechanical working of the pores into the surface, it is possible to go over from round pore shapes to any geometric shapes, for example, longitudinal channels having different profiles. The depth of the pores or the channels or other depressions is dependent on the pore width. The layer thickness of the microstructured surface is preferably in the range from a few microns to a few millimeters, for example in the range from 1 to 10 000 μm, preferably in the range from 10 to 1000 μm.

In one variant of the apparatus of the invention, the entire wall surface of the lines and channels of the cooling circuit which comes into contact with the liquid coolant can be configured as a microstructured surface. However, in a preferred variant, the microstructured surfaces are restricted to regions of the cooling circuit which are located in the region to be cooled in the internal combustion engine and/or in any heat exchangers for cooling hot gases which are arranged in the cooling circuit, for example the abovementioned exhaust gas coolers.

As a liquid coolant, preference is given to using the above-described water-based coolant compositions comprising alkylene glycols. In a preferred variant of the apparatus of the invention, the coolant can comprise surface-active additives, for example surfactants, which reduce the surface tension of the coolant. Such surface-active additives additionally aid the boiling process by further reducing the superheating of the wall required for commencement of bubble boiling.

Cooling circuits of modern internal combustion engines are usually operated at a pressure of from 1.5 to 5 bar absolute, in order to increase the saturation temperature of the liquid coolant and thus improve the cooling action further.

The invention also provides a process for producing the apparatus of the invention for liquid cooling of internal combustion engines. According to the invention, a cooling circuit for a liquid coolant which has cooling channels which are at least partly in thermal contact with the internal combustion engine is constructed, wherein a microstructured surface is produced on at least part of the walls of the cooling channels which come into contact with the liquid coolant.

Processes for producing cooling circuits for internal combustion engines, in particular motor vehicle engines are known to those skilled in the art, so that the following information can be restricted to the production of the microstructured surfaces provided according to the invention. The inner wall of the cooling channel on which the microstructured surface is produced preferably comprises a material which has good thermal conductivity, in particular metal. The channels are particularly preferably formed during casting of the internal combustion engine so that the channel wall usually consists of the same material as the engine block, the cylinder head cover or the crankcase.

In a first variant of the process of the invention, the microstructured surface is produced by mechanical treatment of the inner walls of the cooling channels. For example, suitable microstructures can be produced by cutting machining of the walls, for example, milling of grooves and other depressions, or by embossing of structures by means of appropriately profiled rollers or plates. Suitable microstructured coatings, as can also be used in the apparatus of the invention known, for example, from chemical process engineering. Thus, for example, the company Wieland-Werke AG, Ulm, Germany, produces heat exchanger tubes under the name “enhanced boiling tubes”. Here, microstructures produced mechanically in a targeted manner serve to improve heat transfer during vaporization. Suitable mechanical methods of producing such structures are described, for example in EP-A 0 607 839, DE-C 101 56 374, DE-C 44 04 357 and DE-A-102 10 016.

A microstructuring of the walls which are subjected to high thermal stresses can be effected, for example, by abrasive treatment of the walls, for example, by blasting with sand, metal spheres or ceramic spheres or other abrasive particles. The micro structured surface layer can also be produced by means of chemical treatment of the walls, for example by etching of the wall surface with suitable acids or bases.

In a further variant of the process of the invention, the microstructured surface is produced by a deposition of a rough and/or porous layer on the walls to be treated. The microstructured surface can in this case also consist of a material different from that of the inner wall of the cooling channels. A wide variety of processes known from coating technology can be employed, for example flame spraying, PVD or CVD processes, powder coating or plasma coating, sputtering or various spray or atomization processes. It is also possible to use coatings as are known from the known tubes having a porous coating which are obtainable under the name “High-Flux Tubes”, from UOP LLC, Des Plaines, Ill., USA. There, an improvement in heat transfer during vaporization is achieved by means of randomly distributed pores. A process for producing such porous layers by application of a porous foam and subsequent galvanization of the foam is described in U.S. Pat. No. 4,136,427. Other methods of producing suitable layers are described, for example, in JP-A 2001-038463 or FR-A 0 112 782, in which metal particles having a suitable particle size are joined to one another by means of a solder material to give a porous surface layer. Another possibility is to admix the coolant with an additive which decomposes thermally and thus forms degradation products which are deposited as a porous coating on the cooling surface.

In a particularly preferred variant of the process of the invention, the microstructured surface is produced directly during casting of the internal combustion engine. Here, the casting mold can have been provided with an appropriate microstructure. However, a particularly simple possibility is to coat the surfaces of the shaped bodies for the high spaces of the engine block with a slurry or a slip of metal and/or ceramic particles of appropriate particle size and a polymer which decomposes during casting before casting of the engine block.

The invention is illustrated below by an example shown schematically in the accompanying drawing and by comparative experiments carried out in a boiling test rig.

In the drawing:

FIG. 1 schematically shows an apparatus for liquid cooling of an internal combustion engine;

FIG. 2 shows boiling lines which indicate the aging behavior of a cast tube provided with a microstructured surface according to the invention;

FIG. 3 shows boiling lines of a cast tube according to the invention and an unmodified cast tube in a comparative experiment; and

FIG. 4 shows boiling lines of cast tubes according to the invention and unmodified cast tubes at different flow velocities in a further comparative experiment.

FIG. 1 schematically shows an apparatus 10 according to the invention for liquid cooling of an internal combustion engine 11. In the example shown, the internal combustion engine 11 is configured as a motor vehicle engine which has a cylinder head 12 a and an engine block 12 b with a crankcase. The motor vehicle engine 11 is cooled by means of a coolant which circulates in a cooling circuit 13. The cooling circuit 13 has a pump 14 and an external, air-cooled main heat exchanger 15 which in the case of a motor vehicle is usually referred to as “radiator”. A thermostat valve 17 controlled by a temperature sensor 16 is located upstream of the inlet of the radiator 15 and directs the coolant stream, depending on the operating conditions of the internal combustion engine, either into a large circuit 18 which leads through the heat exchanger 15 or into a small circuit 19 which bypasses the heat exchanger 15.

The coolant stream coming from the main heat exchanger 15 enters the motor vehicle engine 11 via a cooling liquid inlet 20 located in the region of the crankcase 12 b. Depending on the number of cylinders in the engine, the coolant stream is divided into a plurality of substreams in the internal combustion engine and these are conveyed in cooling channels 23, 24 along the outer wall of the combustion chambers 25, 26 into the cylinder head 12 a where the substreams are again combined and conveyed into an outlet line 27 which leaves the motor vehicle engine 11 via the output 28. The line section 29 adjoining the outlet 28 leads the coolant back to the heat exchanger 15 where it gives off the heat taken up in the motor vehicle engine 11 to the environment.

In the sections which are subject to particularly high thermal stresses in the interior of the motor vehicle engine, in particular in the regions 23, 24 surrounding the combustion chambers, the inner walls of the coolant lines or channels are provided with the microstructured surface according to the invention.

In addition, the internal combustion engine 11 shown in FIG. 1 has an exhaust gas recirculation facility which is denoted overall by the reference numeral 30 and comprises an exhaust gas cooler 31. Air is drawn via an intake line 32 into the combustion chambers 25, 26 of the internal combustion engine 11. The exhaust gas formed after combustion of the fuel is discharged via an exhaust gas line 33. A substream of the exhaust gas is branched off at a valve-controlled branch point 34 and conveyed via an exhaust gas return line 35, 36 into the intake line 32, so that the oxygen excess in the combustion chambers is reduced and the combustion temperature is decreased, which leads to a reduction in the NO_(x) loading of the exhaust gases and to a lower fuel consumption. These effects can be reinforced by cooling of the recirculated exhaust gases. For this purpose, an exhaust gas cooler 31 which cools the hot exhaust gases is located in the exhaust gas return line 35, 36. The exhaust gas cooler 31 can have a separate cooling circuit. However, in the embodiment depicted, a substream of the cooling circuit 13 is branched off at a valve-controlled branch point 37 and conveyed via a line 38 to the exhaust gas cooler 31. The heated coolant is subsequently recirculated via a line 39 to the coolant circuit 13. The exhaust gas cooler 31 can, for example, be configured as a shell-and-tube heat exchanger with the exhaust gas stream being divided among individual tubes 40 around which the coolant 41 flows. The outer walls of the tubes 40 are provided with the microstructured surface layer according to the invention.

Further features of the cooling circuits of modern motor vehicle engines which are known per se to those skilled in the art, e.g. pressure devices, secondary heat exchangers which are in thermal contact with the heating system of the passenger compartment, etc. have been omitted in the schematic depiction in FIG. 1 for reasons of clarity.

Comparative Experiments:

To test the effectiveness of the microstructured surface layer provided according to the invention, conventional cast tubes made of gray cast iron (cast iron comprising 3.5% of C, 2.0% of Si, 0.7% of Mn and 0.5% of phosphorus as significant alloying elements) and having an untreated casting skin were compared in a boiling test rig with a similar cast tube to whose casting skin a microstructured surface layer according to the invention had been applied. For this purpose, a porous layer of an iron alloy (Cr. about 29%, Ni about 6%, B about 3% , balance: iron) was applied in a layer thickness of about 200-400 μm by metal spraying using compressed air. The current for melting the iron wires was about 150 A at about 40 volts. The molten metal was distributed by means of about 4 bar compressed air over the surface of the boiling tube. Coating of the above 4 cm long tube having a diameter of about 1 cm was complete after a coating time of about 10 seconds.

The effectiveness of heat transfer was determined as a function of temperature, cooling medium used and flow velocity by means of boiling lines in a boiling test rig. Boiling lines described, for single- or two-phase heat transfer, the relationship between the heat flow transferred per unit area (heat flux) and the wall temperature or the difference between wall temperature and saturation temperature of the liquid (known as the superheating of the wall T_(W)-T_(sat)).

The radiator protection product “Glysantin® Alu Protect” marketed commercially by the applicant was employed without antifoam for the measurements carried out using the coated cast tube. In a comparative experiment using an uncoated cast tube, the radiator protection product “Glysantin® Protect Plus” marketed commercially by the applicant was also employed. The system pressure in all cases examined was p_(sys)=3.2 bar absolute, and the temperature of the cooling medium was kept constant at T_(sys)=100° C.

The typical shape of boiling lines can be described as folllows: at wall temperatures below the saturation temperature and at low superheating of the wall, heat transfer occurs by free, single-phase convection, which as the temperature difference increases leads to a better heat transfer coefficiency and thus to a gentle rise in the boiling line. Depending on the wettability of the wall, the first vapor bubbles are formed at particular places on the wall surface after a more or less pronounced boiling delay, and the number and size of these increase with increasing superheating of the walls. After detachment of the first bubbles from the contact surface, bubble boiling commences. In this region, the contact surface is still completely wetted by the liquid. As a result of the increased production of vapor and the intensive stirring action of the coalescing vapor bubbles, the heat flux increases steeply.

1. Aging Behavior

-   -   The first study related to the aging behavior of the surface and         the changes in heat of vaporization transfer which are generally         associated therewith. For this purpose, a plurality of boiling         lines were recorded over a period of 28 hours at a mean flow         velocity u_(b)=0.25 m/s.     -   The result of the experiment is shown in FIG. 2.     -   It can clearly be seen from the boiling lines recorded over this         period of time that virtually no aging, i.e. no deterioration in         the boiling behavior, is observed since the individual boiling         lines at various aging states virtually coincide to give a         simple line.

2. Comparison of Coated Cast Tube with Uncoated Cast Tube

-   -   In the graph of FIG. 3, the boiling behavior of the uncoated         cast tube using “Glysantin® Protect Plus” (curve A1 in FIG. 3)         and using “Glysantin® Alu Protect” (curve A2) as coolant is         compared with that of the coated cast tube using “Glysantin® Alu         Protect” (curve B1) as coolant. All boiling lines examined here         have the same aging state. The comparison is once again carried         out for the case of the medium flow velocity of u_(b)=0.25 m/s.     -   The improved heat of vaporization transfer of the cast tube         having a porous surface compared to that of the standard cast         tube can be seen clearly in FIG. 3. This improvement in the heat         of vaporization transfer is reflected at high heat fluxes (≈400         000 W/m²) in a reduction in the surface temperature T_(W)         compared to the standard cast tube by an amount of         ΔT_(W)=15-20° C. More precise examination of the temperature         range in which boiling commences shows that in the case of the         modified cast tube having a porous surface, boiling commences at         a wall temperature T_(W)<T_(sat) (deviation of the boiling line         from linearity).

3. Variation of the Flow Velocity

-   -   The graph of FIG. 4 shows boiling lines of the uncoated cast         tube (set of curves Ai) and the coated cast tube (set of curves         Bi) at various mean flow velocities u_(b).     -   In all cases shown, the temperature of the cooling medium         T_(sys)=100° C. and the absolute system pressure p_(sys)=3.2         bar. As cooling medium, “Glysantin® Alu Protect” was used in         each case. The indices i of the set of curves Ai or Bi in each         case refers to flow velocities u_(b) of 0.1 m/s (i=1), 0.25 m/s         (i=2), 0.5 m/s (i=3), 0.75 m/s (i=4), 1.0 m/s (i=5) and 1.5 m/s         (i=6).     -   The distinct improvement in both the single-phase and the         two-phase heat transfer when using the modified cast tube having         a porous surface compared to the uncoated cast tube can be seen         in all cases examined.

In summary, the result of the comparative experiments is:

-   -   It can clearly be seen from the boiling lines recorded using the         modified cast tube that virtually no aging of the surface, i.e.         no deterioration in the boiling behavior is observed.     -   Owing to the greater surface roughness, a distinct improvement         in the single-phase heat transfer when using the test specimen         having a porous surface compared to the standard tube was         observed.     -   Both the readiness with which boiling occurs and the boiling         activity could be increased significantly when using the porous         surface compared to the standard surface having an untreated         casting skin. As a result, the surface temperature could be         reduced by about 15° C. at high heat flows. 

1. An apparatus for cooling an internal combustion engine (11) comprising a cooling circuit (13) which comprises at least one cooling channel (23, 24, 41) for a liquid coolant which is in thermal contact with at least one component (12 a, 12 b, 31) of the internal combustion engine (11), wherein a wall of the cooling channel (23, 24, 41) which comes into contact with the coolant has a microstructured surface at least in a subregion.
 2. The apparatus according to claim 1 wherein the microstructured surface has a mean surface roughness Ra in the range from 1 to 1500 μm.
 3. The apparatus according to either claim 1 or 2, wherein the microstructured surface has a porous structure.
 4. The apparatus according to claim 3, wherein the mean size of the pores of the microstructured surface is in the range from 1 to 500 μm.
 5. The apparatus according to either claim 3 or 4, wherein the proportion of pores is in the range from 1 to 90%.
 6. The apparatus according to any of claims 3 to 5, wherein the microstructured surface having a porous structure has a layer thickness of from 1 to 10 000 μm.
 7. The apparatus according to any of claims 3 to 6, wherein the structures of the microstructured surface are arranged regularly.
 8. The apparatus according to any of claims 3 to 6, wherein the structures of the microstructured surface are arranged randomly.
 9. The apparatus according to any of claims 1 to 8, wherein at least the sections of the wall or walls of the cooling channel which are subjected to high thermal stress in operation have the microstructured surfaces.
 10. The apparatus according to any of claims 1 to 9, wherein the component of the internal combustion engine (11) which is in thermal contact with at least one cooling channel (23, 24, 41) is a cylinder head (12 a) and/or at least a crankcase (12 b) of the internal combustion engine and/or an exhaust gas cooler (31).
 11. The apparatus according to any of claims 1 to 10, wherein an aqueous coolant which comprises surface-active additives, in particular surfactants, circulates in the cooling circuit.
 12. A process for producing an apparatus for cooling an internal combustion engine, in which a cooling circuit for a liquid coolant which has cooling channels which are at least partly in thermal contact with the internal combustion engine is constructed, wherein a microstructured surface is produced on at least part of the walls of the cooling channels which come into contact with the liquid coolant.
 13. The process according to claim 12, wherein the microstructured surface is produced by mechanical and/or chemical treatment of the walls.
 14. The process according to claim 12, wherein the microstructured surface is produced by application or deposition of a coating material on the walls.
 15. The process according to claim 12, wherein the microstructured surface is produced during casting of the internal combustion engine. 